Inverters of this type are used, for example, in conjunction with a building-integrated photovoltaic system. The energy generated by a photovoltaic (PV) generator of the photovoltaic system in the form of direct current is converted by the inverter into grid-compliant alternating current and fed into the plurality of (usually three) phases of the local energy distribution grid, which in this context is also referred to as building installation. In this case, an inverter is also understood to mean a plurality of conversion apparatuses which jointly feed into a local energy distribution grid.
The energy generated by the photovoltaic generator can thus be used, via the building installation, for supplying local consumers. Excess energy is fed into the superordinate energy supply grid from the local energy distribution grid at a grid transfer point. Conversely, energy flows out of the superordinate energy supply grid into the local energy distribution grid if the power demand of consumers in the local energy distribution grid exceeds the electric power provided by the local energy generation device.
As an alternative or in addition to the local energy generation device, a local energy storage device, for example a battery, can be provided, wherein, similarly to the case of the energy generation device, energy can be output via the inverter into the local energy supply grid. Arrangements comprising such an energy storage device are used for at least temporarily ensuring the energy supply in the local energy distribution grid even in the event of failure of the superordinate energy supply grid. They can also be used to be able to control a withdrawal of energy from the superordinate energy supply grid depending on parameters such as the energy price or the availability of energy.
In the simplest and conventional case, an identical power is introduced into the individual phases of the local energy distribution grid by the inverter. Since, however, the consumers connected to different phases of the local energy distribution grid or possibly additionally existing (possibly single-phase) generation installations generally do not load the phases uniformly, however, this results in an unsymmetrical loading situation of the phases of the superordinate energy supply grid at the grid transfer point. The document EP 2 348 597 A1 discloses, in order to prevent such a load imbalance at the grid transfer point, the determination of the power or current flow on the individual phases at the grid transfer point and the operation of an inverter such that said inverter does not feed the power generated by a local energy supply device uniformly into the phases of the local energy distribution grid, but such that a feed-in or withdrawal situation results at the grid transfer point which is as symmetrical as possible.
Furthermore, it is known to design and operate inverters such that, in the event of a failure of the superordinate energy supply grid, the locally generated and/or stored energy is fed into the individual phases of the local energy distribution grid as required in order to ensure operation of the consumers in the local energy distribution grid.
With the mentioned method, it is not possible to ensure a supply to the consumers connected to the local energy distribution grid which lasts as long as possible if not all, but only some, of the phases, for example one or two of three phases of the superordinate energy supply grid, have a failure.
An object of the present invention consists in providing an inverter or an operating method for an inverter or an energy supply installation comprising an inverter, in which a supply to local consumers in a local energy distribution grid can also be provided when some of the phases of a polyphase energy supply grid have a failure, i.e. there is a so-called partial island situation.
This object is achieved by an inverter, an operating method for an inverter and an energy supply installation having the respective features of the independent claims. Advantageous configurations and developments are the subject matter of the dependent claims.
An inverter according to the invention of the type mentioned at the outset is characterized by the fact that it comprises a control connection for connection to the switching device such that individual phases of the local energy distribution grid are connectable to corresponding phases of the energy supply grid or are disconnectable from one another selectively via the control connection, and the inverter is configured to disconnect, in the event of a grid fault on at least one, but not all, of the phases of the energy supply grid, the at least one faulty phase of the energy supply grid from the corresponding phase of the local energy distribution grid via the control connection, and to apply a grid-compliant AC voltage to the at least one disconnected phase of the local energy distribution grid.
If a partial island situation occurs, i.e. if a fault occurs on one or more, but not all, of the phases of the superordinate energy supply grid, the local energy distribution grid can be decoupled, with respect to this phase or these phases, from the energy supply grid and thereupon grid-compliant and correct, in particular also in respect of the phase angle, alternating current can be applied to said local energy distribution grid by the inverter, so that the consumers connected to this phase in the local energy distribution grid can continue to be operated as far as possible without any interruptions.
In preferred configurations of the inverter, in this case the switching device and/or a grid monitoring device, which is configured for the selective identification of the grid fault on each individual one of the phases of the energy supply grid, are integrated in the inverter. In this way, a compact system design is achieved. In a further preferred configuration of the inverter, a signal connection for connection to an external grid monitoring device is provided. This is advantageous for being able to use a grid monitoring device which may already be in existence.
In further preferred configurations of the inverter, said inverter is configured to use preferably power provided by the energy generation device and/or the energy storage device and/or power drawn from a non-disconnected phase of the local energy distribution grid for application to the at least one disconnected phase of the local energy distribution grid. In all cases, the ability of the inverter to provide AC voltage with the required phase angle is utilized in order to continue to operate the consumers on the disconnected phase(s). In this case, locally generated or stored energy can be used or, for example, if insufficient such energy is available, energy which is drawn from other, non-faulty phases of the energy supply grid can also be used. Within the scope of the application, “continued operation” of the consumers is understood to mean that energy can be supplied to said consumers for a period of time which is markedly longer than the period of time of a grid period and, for example, is in the region of a few seconds and preferably a few minutes or longer.
Generally, a DC link having an arrangement of (buffer) capacitors is connected upstream of inverter bridges in order to smooth the DC voltage provided by the DC source despite the pulsed current consumption taking place during conversion into alternating current and, as a result, to increase the maximum peak current pulse that can be withdrawn. The DC-link/capacitor arrangement is therefore used for buffer-storing energy within a grid period, wherein the capacitance of such a DC-link/capacitor arrangement is insufficient for a temporary continued operation of the consumers within the meaning of the application. Within the scope of the application, a DC-link/capacitor arrangement is therefore not an energy storage device which is suitable for the continued operation of the consumers.
A method according to the invention for operating an inverter has the following steps: monitoring is performed to ascertain whether there is a grid fault in at least one phase, but not all of the phases, of the energy supply grid. If there is a grid fault, the switching device is actuated and the at least one faulty phase of the energy supply grid is decoupled from the corresponding phase of the local energy distribution grid. Then, grid-compliant AC voltage is applied to the at least one disconnected phase of the local energy distribution grid by means of the inverter. This results in the advantages already described previously in connection with the inverter.
In an advantageous configuration of the method, power required for application to the at least one disconnected phase of the local energy distribution grid is drawn from the energy generation device and/or from the energy storage device and is converted from direct current into alternating current by the inverter. Preferably, in this case excess power of the energy generation device can be fed by the inverter into non-disconnected phases of the local energy distribution grid.
In an advantageous configuration of the method, missing power for application to the at least one disconnected phase of the local energy distribution grid is drawn by the inverter from at least one non-disconnected phase of the local energy distribution grid and fed into the at least one disconnected phase. Preferably, the transmission of power from the at least one non-disconnected phase into the at least one disconnected phase of the local energy distribution grid takes place via a DC link of the inverter. In this case, therefore, there is first rectification of current from a non-disconnected phase and then conversion into alternating current for the disconnected phase. The capacity of the inverter to set a desired phase angle at an output is thus utilized in order to apply grid-compliant AC voltage to the disconnected phase, even in respect of the phase angle.
In further advantageous configurations of the method, the withdrawal of power from the energy storage device is controlled in such a way that the loading of the non-disconnected phases remains below a loading threshold value, in particular below a trigger threshold of a fuse. The method according to the invention provides the advantage that, in order to supply the consumers of the disconnected phases, the required power can be drawn, mixed, from the various mentioned sources. The withdrawal of power from the energy storage device can then advantageously be used, as required, in order to prevent overloading of the non-disconnected phases. In further advantageous configurations, other criteria can be taken into consideration as an alternative or in addition in order to control the distribution of the power among the various sources. Thus, the proportion of the power which is drawn from the non-disconnected phases can be determined depending on the state of charge of the energy storage device or the power capacity of the energy generation device or the loadability of the power sections of the inverter which are assigned to the non-disconnected phases, respectively.
An energy supply installation according to the invention is polyphase and comprises at least one inverter, at least one energy generation device and/or an energy storage device, and a switching device, via which the energy supply installation can be coupled to a likewise polyphase superordinate energy supply grid, and a grid monitoring device. The energy supply installation is characterized by the fact that a control device is provided, which is configured to actuate the switching device and/or the inverter, depending on signals from the grid monitoring device, in order to implement one of the abovementioned methods. In this case, too, the advantages already mentioned above are achieved. In this case, a polyphase inverter in which the mentioned control device is possibly integrated can be used. However, it is also possible for the control device to be a separate component of the energy supply installation, which separate component correspondingly actuates a polyphase inverter or else a plurality of single-phase inverters in order to implement the method according to the invention.
FIG. 1 shows an energy supply installation for supplying electrical consumers in a building in the form of a block circuit diagram. The figure shows a building 1 having a local energy distribution grid 2, via which current is supplied to consumers 3, 4. The local energy distribution grid 2 is in this case illustrated, by way of example, as a three-phase energy supply grid comprising phases L1, L2 and L3 and a neutral conductor N. An optional PE conductor is not indicated for reasons of clarity. Single-phase consumers 3 and in this case, by way of example, a three-phase consumer 4 are connected to the energy distribution grid 2, distributed in the building 1.